The movement of the earth around the sun in a fixed path or orbit is called Revolution.

The axis of the earth which is an imaginary line, makes an angle of 66 1/2o with its orbital plane. The plane formed by the orbit is known as the orbital plane.

The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis.

The earth takes about 24 hours to complete on rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.

What would happen if the earth did not rotate?

The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to the region. The other half would remain in darkness and be freezing cold all the time. Life would not have been possible in such extreme conditions.

Revolution:

It takes 365 ¼ days (one year) to revolve around the sun.

A year is usually divided into summer, winter, spring and autumn seasons.

Seasons change due to the change in the position of the earth around the sun.

The longest day and the shortest night at Tropic of Cancer occur on 21st At this time in the Southern Hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.

On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn, a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights. The reverse happens in Northern Hemisphere. This position of the earth is called the Winter Solstice.

On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the pole is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.

Hence, there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.